Egyptian police officers and firefighters gather at the Egyptian police headquarters after a blast in downtown Cairo on Friday. / Khalil Hamra, AP

by Sarah Lynch, Special for USA TODAY

by Sarah Lynch, Special for USA TODAY

CAIRO â?? Egypt's capital was hit Friday with four bomb attacks, one of which ripped through police headquarters and killed four people, on the eve of the three-year anniversary of the revolution that toppled dictator Hosni Mubarak.

The massive explosion that struck Egyptian police headquarters in downtown Cairo early Friday was the deadliest bomb blast in Cairo since the Egyptian military arrested Islamist president Mohamed Morsi and banned his party from office.

Strips of the police headquarters' faÃ§ade were damaged in the blast, which also blew out the building's windows. The explosion was believed to be from a car bomb, according to the state new agency MENA.

In a separate, smaller attack, a home-made bomb exploded across the Nile River in Dokki, the state news agency said. One was killed and eight were wounded in the attack. A third bomb went off near a police station in Giza, the state news agency reported. No casualties were immediately reported.

Later in the day, a fourth attack killed one person after officials said a bomb planted on a road hit a convoy of security forces, the Associated Press reported. The blast brings the toll from the series of explosions in Cairo to six.

The blasts come a day before manyb Egyptians planned to celebrate the anniversary of the uprising against Mubarak, president of Egypt for three decades. Hundreds of thousands of people took to the streets to protest against him in early 2011.

Six months ago, Morsi was the second Egyptian leader to be ousted in 2 Â½ years. Morsi was ousted amid massive protests against his rule following edicts he imposed that opponents saw as returning Egypt to a dictatorship.

Morsi was ousted by Army Gen. Abdel Fattah el-Sisi, whom Morsi had appointed, and the government is being been run by a transitional panel appointed by Sisi. The panel says it is planning for new elections this year; Morsi's Muslim Brotherhood party is banned from running.

Earlier this month Egyptians overwhelmingly approved a new constitution to replace one that Morsi's allies in parliament pushed through, a document that opponents said was unfair to minorities and opened the door to harsh Islamic rule.

The Obama administration has criticized the overthrow of Morsi, who was the country's first freely elected president, and withheld military aid. On Friday the U.S. Embassy condemned the terrorist attacks.

"We fully support the Egyptian government's efforts to bring the perpetrators of these crimes to justice," the embassy said in a statement on its website. "The Embassy extends its deepest condolences to the families and friends of the victims, and we hope for the quick and full recovery of the injured."

Interim President Adly Mansour called the attacks cowardly acts that aim to weaken the resolve of the Egyptian people.

"These attacks only increase our perseverance and unwavering commitment to eradicate terrorism from the whole of Egypt and move forward toward implementing the road map for the future," the Egyptian presidency said in a statement. "We will not hesitate to undertake the necessary measures to preserve the lives of our people, and the security of our homeland."

Several blasts have rocked Cairo since Morsi was overthrown in August. A car bomb targeted Egypt's interior minister last summer, although he was unharmed. Last week, an explosion damaged a court complex in North Giza just before polls opened in a vote on a proposed new constitution.

Attacks have also occurred elsewhere across the country. In December, at least 16 people were killed in an explosion at a police headquarters in Mansoura, in Egypt's Nile Delta region. Ansar Beit al-Maqdis, an al-Qaeda inspired group, claimed responsibility for the attack, local news media reported.

Egypt's Sinai Peninsula, which shares a border with Israel and the Gaza Strip, has been home to regular attacks against security and government outposts amid growing militant activity in the restive region.

In recent weeks, Egyptian authorities escalated a broadening crackdown on opposition, claiming it is trying to fight terrorism. In December, Egyptian authorities declared the Muslim Brotherhood a terrorist group after imprisoning many of its members and leaders.

The Brotherhood has repeatedly denied that it has used violence in its opposition to the government, which it claims took power by force last August and is therefore illegitimate.

"The National Pro-Legitimacy Alliance condemns the #CairoBomb & reaffirms commitment to peaceful struggle against the coup," a Brotherhood-led, anti-coup group said on Twitter.

On Saturday, anti-government protests and rival celebrations marking the 2011 uprising against Mubarak are expected to take place across the capital amid heightened security, raising prospects for clashes and more violence.

Ahead of the weekend's demonstrations, the U.S. Embassy in Cairo warned citizens to remain alert and review plans for personal security.

"U.S. citizens are advised to elevate their level of awareness as we move into the weekend and limit their movements on Saturday, January 25 to the near vicinity of their neighborhoods," a security message for U.S. citizens said.